1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
Conventional spark plugs for internal combustion engines such as automobile engines include those in which a chip formed from a noble metal alloy is welded to a distal end portion of a ground electrode. An exemplary material used to form the noble-metal chip is a noble-metal alloy which contains platinum (Pt) as a main component. Also, for example, addition of rhodium (Rh), whose melting point is higher than that of Pt, to a Pt alloy has been proposed as a measure for enhancing resistance to spark consumption (refer to, for example, Patent Document 1).
Furthermore, projecting a noble-metal chip from an electrode and using a noble-metal chip having a reduced diameter have also been proposed as measures for enhancing ignition and spark propagation (refer to, for example, Patent Document 2).
As mentioned above, various measures have been adopted in order to obtain spark plugs having excellent resistance to spark consumption, excellent ignition performance, etc. However, these measures are premised on having a reliable joint between a noble-metal chip and an electrode. In order to meet this need, a technique has been proposed for reliably joining the noble-metal chip and the electrode by laser welding such that the noble-metal chip and the electrode are fused together to form a weld portion (refer to, for example, Patent Document 3).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 58-198886
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2001-345162
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2005-93221
3. Problems to be Solved by the Invention
Meanwhile, in a spark plug having the above-mentioned weld portion, the presence of a relatively large hole called a void in the weld portion deteriorates the mechanical strength of the weld portion. Therefore, generally, the absence of a void or the like in the weld portion as shown in FIG. 6 is desirable.
However, when subjected to severe operating conditions of present day internal combustion engines, even in a spark plug whose weld portion is completely free of a void or the like, the chip may suffer some separation or become disjoined from the electrode. Particularly, in recent years, in order to enhance heat resistance and corrosion resistance, a nickel (Ni) alloy has been employed to form a ground electrode. In such a case, the stress induced by expansion and contraction in a radial direction of the chip differs among the ground electrode and the noble-metal chip. Strain caused by the difference in induced stress is apt to arise in a boundary region between the ground electrode and the noble-metal chip. Also, in association with the recent tendency toward increased projection length and reduced diameter of the noble-metal chip for enhancing ignition performance and flame propagation performance, the strain caused by the thermally induced stress difference is becoming more marked. Accordingly, for example, as shown in FIG. 7, separation may arise at the interface between the noble-metal chip and the weld portion, and, consequently, the chip may become disjoined.